Calcium plays a vital role in cell function and survival. For example, calcium is a key element in the transduction of signals into and within cells. Cellular responses to growth factors, neurotransmitters, hormones and a variety of other signal molecules are initiated through calcium-dependent processes.
Virtually all cell types depend in some manner upon the generation of cytoplasmic Ca2+ signals to regulate cell function, or to trigger specific responses. Cytosolic Ca2+ signals control a wide array of cellular functions ranging from short-term responses such as contraction and secretion to longer-term regulation of cell growth and proliferation. Usually, these signals involve some combination of release of Ca2+ from intracellular stores, such as the endoplasmic reticulum (ER), and influx of Ca2+ across the plasma membrane. In one example, cell activation begins with an agonist binding to a surface membrane receptor, coupled to phospholipase C (PLC) through a G-protein mechanism. PLC activation leads to the production of inositol 1,4,5-triphosphate (IP3), which in turn activates the IP3 receptor causing release of Ca2+ from the ER. The fall in ER Ca2+ then signals to plasma membrane store-operated calcium (SOC) channels.
Store-operated calcium (SOC) influx is a process in cellular physiology that controls such diverse functions such as, but not limited to, refilling of intracellular Ca2+ stores (Putney et al. Cell, 75, 199-201, 1993), activation of enzymatic activity (Fagan et al., J. Biol. Chem. 275:26530-26537, 2000), gene transcription (Lewis, Annu. Rev. Immunol. 19:497-521, 2001), cell proliferation (Nunez et al., J. Physiol. 571.1, 57-73, 2006), and release of cytokines (Winslow et al., Curr. Opin. Immunol. 15:299-307, 2003). In some nonexcitable cells, e.g., blood cells, immune cells, hematopoietic cells, T lymphocytes and mast cells, SOC influx occurs through calcium release-activated calcium (CRAC) channels, a type of SOC channel.
The calcium influx mechanism has been referred to as store-operated calcium entry (SOCE). Stromal interaction molecule (STIM) proteins are an essential component of SOC channel function, serving as the sensors for detecting the depletion of calcium from internal stores and for activating SOC channels.